What is the Henderson- Helbach equation?

In chemistry, the Henderson-Helbalch (often misspelled as Henderson-Helbach) equation describes the derivation of pH as a measure of acidity (using pKa, the acid dissociation constant) in biological and chemical systems. The equation is also useful for estimating the pH of a buffer solution and finding the equilibrium pH in acid-base reactions (it is widely used to calculate isoelectric point of the proteins).